This invention relates to an electrolysis process for producing metallic aluminum from aluminum sulfide and to a molten bath in which the electrolysis is carried out.
Traditionally, metallic aluminum has been produced by the established Hall-Heroult process in which aluminum oxide is electrolyzed in a cryolite bath to the metallic form. Typical conditions include a voltage of about 4.5 volts, a current efficiency of about 90%, and an operating temperature of about 900.degree.-1000.degree. C. In the process, carbon anodes are consumed and require periodic replacement. Since this process has required large quantities of energy to carry out the electrolysis and provide the high operating temperatures, increasing costs for electrical power and other forms of energy have caused economic difficulties to aluminum producers.
Efforts have been made by various organizations to develop alternative processes with reduced energy requirements. One process utilizes anhydrous aluminum chloride in place of aluminum oxide in the electrolysis bath and operates at a reduced voltage of about 3.3 volts (at current density of about 0.8 A/cm.sup.2), a current efficiency in the order of about 90% and an operating temperature of about 750.degree. C. In this process, the electrolysis of aluminum chloride is carried out in a chloride melt containing one or more alkali metal chlorides to produce the metallic aluminum. Additives for particular purposes such as magnesium chloride in an amount of about 1 mol % (as reported in U.S. Pat. No. 3,755,099) may also be present. It has been estimated that this process provides an electrical energy saving in the order of about 30% over the same costs for the traditional process using aluminum oxide.
Although these savings in electrical energy have been impressive, the requirement of synthesizing about 5 kg of anhydrous aluminum chloride per kg of aluminum makes new processes with reduced energy requirements desirable, particularly in view of the overall large energy requirement. In the efforts to develop new processes, other sources of aluminum such as aluminum sulfide where only 2.8 kg of sulfide is required per kg of aluminum have been evaluated. While aluminum sulfide may be electrolyzed to form metallic aluminum, previous experimental efforts to carry out the process have not been characterized by satisfactory results. In some instances with baths containing fluoride melts from molten cryolite, current efficiencies have been in the order of 5%. With baths containing alkali metal chloride melts, the aluminum sulfide is only slightly soluble which limits the usefulness of the process. In addition, some experiments have resulted in products other than metallic aluminum or elemental sulfur. Therefore, an improved electrolysis process and an improved bath for producing metallic aluminum from aluminum sulfide are desirable.
One object of this invention is an electrolysis process utilizing aluminum sulfide as a source of aluminum. A second object is a process for producing metallic aluminum with reduced energy requirements. Another object is a process utilizing a bath with improved solubility for aluminum sulfide. Still another object is an aluminum sulfide process with improved current efficiencies in the order of at least 70%. Yet another object is an aluminum sulfide process with higher current densities. An additional object is an aluminum sulfide process with reduced problems from undesirable products. Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.